Let's do some math ..  Rounding examples of 7 random hypothetical purchases:
  Case Tag  
Price  
Sales  
Tax %  
Raw Total  
Incl. Sales Tax  
Rounded  
to Penny  
Rounded  
to Nickel  
Rounded  
to Dime  
  Purchase #1 $143.31   10.333%   $158.1182   $158.12   $158.10   $158.10  
  Purchase #2 $219.84   9.726%   $241.2216   $241.22   $241.20   $241.20  
  Purchase #3 $99.68   5.164%   $104.8275   $104.83   $104.85   $104.80  
  Purchase #4 $23.69   7.215%   $25.3992   $25.40   $25.40   $25.40  
  Purchase #5 $9.35   9.277%   $10.2174   $10.22   $10.20   $10.20  
  Purchase #6 $89.90   4.503%   $93.9482   $93.95   $93.95   $93.90  
  Purchase #7 $9.74   13.470%   $11.052   $11.05   $11.05   $11.10  
  7 purchase totals = $595.51       $644.7841   $644.79   $644.75   $644.70